This invention relates to the fabrication of semiconductor devices having low resistance contacts and, more particularly, to the construction of a thermally stable non-alloyed contact containing small amounts of indium to yield a low resistance path to electric current.
Semiconductor devices frequently employ low resistance contacts to provide an electrically conducting path between semiconductor material and an electric circuit external to the semiconductor material. A field-effect transistor (F E T ) is a common example of a semiconductor device having low resistance contacts. The low-resistance transistor contacts are of the form known as ohmic contacts and are used in the formation of the source and the drain terminals of the transistor. The low resistance contact forms a path to the semiconductor and allows electrical current to flow into or out of the semiconductor without significant voltage drop.
A semiconductor material of particular interest herein is gallium arsenide (GaAs). Low resistance contacts to GaAs and related III-V compound semiconductors are essential for the fabrication of many high performance optical, microwave and logic devices. A commonly used low resistance contact to n-GaAs is composed of an alloyed gold-nickel-germanium (Au-Ni-Ge) system which produces contact resistance in the range of 0.2 to 1.0 ohm-millimeters. Other materials frequently used in the construction of contacts to GaAs are tungsten silicide and layers of gold, titanium and platinum.
In the manufacture and packaging of a semiconductor device, such as the foregoing FET, after formation of low resistance contacts and non-ohmic gates, the devices are required to withstand an elevated temperature, typically 400.degree. C., for periods of a few minutes to several hours. Such heating over a sustained interval of time increases the resistance. For example, the resistance of a contact of Au-Ni-Ge has been observed to increase by a factor of three during annealing of the contact for 5 hours at 400.degree. Centigrade.
In the construction of a low resistance contact, the resistance is optimized by appropriate selection of material composition and by the cleaning of the substrate prior to the deposition of the material used in the low resistance contact. While presently available techniques may be employed to provide low resistance contacts with acceptably low values of resistance, the above noted problem of increase in resistance with prolonged heating limits the application of currently used low resistance contacts.